The present invention relates generally to a motor grader and a control system for a motor grader of the type in which the vertical position of the grader blade is controlled by a pair of hydraulic cylinders. More particularly, the present invention relates to such a motor grader and control system in which the extension or retraction of one hydraulic cylinder of the pair follows that of the other hydraulic cylinder when vertical movement of the blade is to be effected, thereby maintaining the cross slope angle.
Motor graders are used to alter the contours of construction sites and road beds. Motor graders typically include a work implement, such as a surface-altering blade, that is movably connected to a frame of the grader by a structure including a pair of independently controlled hydraulic cylinders. The hydraulic cylinders are mounted on either side of the machine frame of the motor grader. The hydraulic cylinders are operated independently, and can be extended or retracted to lower or raise the respective ends of the blade relative to the corresponding sides of the machine frame. The blade height may be controlled manually or automatically.
In the manual mode, the operator controls the vertical position of one or both ends of the blade through a pair of control levers mounted in the cab of the motor grader. Each control lever modulates a corresponding control valve that is mechanically linked or electrically linked to the control lever. The manual control valves are connected between an hydraulic fluid supply and the hydraulic cylinders. The operator actuates the manual control valves to achieve the desired elevational position of the blade at one or both ends of the blade. In one variation of manual control, an inclinometer mounted for movement with the blade, provides an inclinometer output indicating the inclination of the blade along its length. The operator sets the desired inclination in the control system. The control system compares the measured inclination with the desired inclination and controls one control valve to extend or retract its associated cylinder at the same time that other cylinder is extended or retracted. This is accomplished while maintaining the slope of the blade at a desired inclination.
In the automatic mode, raising and lowering the blade by retracting and extending the cylinders is controlled based on the difference between desired blade height and measured blade height. The blade height may be measured in any of a number of ways, including the use of GPS-based and laser-based systems. The height of each end of the blade may be measured in this mode of operation. Alternatively, the height of one end of the blade may be measured, and the hydraulic cylinder at that end of the blade operated, based on the difference between the measured and desired height. The hydraulic cylinder at the other end of the blade is actuated based on the difference between the desired and measured inclination of the blade along its length. An inclinometer on or mechanically linked to the blade provides the measurement of blade inclination in this mode, as well.
Inclinometers are typically mechanically or electrically damped to reduce the rapid fluctuations and signal noise in the output that would otherwise result from vibration in the mechanical systems. However, damping produces a time delay in an inclinometer output in a motor grader system that degrades the operation of the system when a change in the elevation of the blade is to be effected. Because the hydraulic cylinder that is controlled manually or automatically is extended or retracted quickly in such a situation, the inherent time delay in the inclinometer results in one end of the blade being lowered or raised by a significant amount before the inclinometer provides an output indicating a change in inclination. This, in turn, results in an erroneous cross slope cut during those times when elevational change is effected.
It is seen, therefore, that there is a need for a motor grader, and control system for a motor grader, capable of changing the vertical blade position without introducing errors in the cross slope orientation of the motor grader blade.